This invention relates to a process for producing a hydrocarbon product from a feed gas and more particularly to a process for producing a hydrocarbon product or methanol from a feed gas stream comprising hydrogen and carbon monoxide.
The carbon monoxide conversion or shift reaction may be written as follows: EQU CO+H.sub.2 O=CO.sub.2 +H.sub.2
The reaction is exothermic, such that an increase in temperature decreases the conversion of carbon monoxide to carbon dioxide.
The carbon monoxide conversion or shift reaction plays a part in a number of chemical processes including those involving reactions between hydrocarbons such as methane or propane and steam in the production of hydrogen. For example, in one method of producing hydrogen from methane, or propane, methane or propane vapors are mixed with steam and catalytically reformed at high temperatures to produce a gas comprising hydrogen, carbon monoxide, carbon dioxide, and any unconsumed reactants. The gas is cooled and the partial pressure of the water vapor is increased by the addition of steam. The resultant mixture is passed over a catalyst in a carbon monoxide shift converter, where a substantial percentage of the carbon monoxide is converted to carbon dioxide with more hydrogen being produced as a result of the CO shift reaction.
The synthesis reaction also plays a part in chemical processes employing synthesis gas, which is generally made up of hydrogen, oxides of carbon, and possibly other constituents such as nitrogen. Synthesis gas may be produced by a variety of methods, including steam hydrocarbon reforming, auto reforming, and the gasification of coal. The resulting synthesis gas is useful in a variety of processes, including the production of methane, methanol, hydrogen, ammonia and other products However, efficiency often requires adjustment of the relative amount of hydrogen to carbon monoxide present in a given system at various stages. For example, in the manufacture of methanol from a synthesis gas comprising hydrogen and the oxides of carbon the ratio of hydrogen to the oxides of carbon must generally be adjusted to provide a favorable stoichiometric relationship. This adjustment is accomplished by use of the carbon monoxide shift reaction in a high temperature shift reactor supplied with steam, followed by the removal of excess carbon dioxide in an acid gas treating unit.
Similarly, in the production of a synthetic natural gas comprising methane from carbon monoxide and hydrogen, the production of methane is generally favored by particular hydrogen to carbon monoxide stoichiometric ratios However, many synthesis gases produced from coal gasification or other processes contain too little hydrogen in relation to the carbon monoxide present to be directly effective in the production of methane. Consequently, the feed gas is generally adjusted by using the carbon monoxide shift reaction on part or all of the synthesis feed gas followed by removal of excess carbon dioxide, if appropriate, to provide the desired ratio of feed components to the methanation process.
A number of processes disclosed in the literature attempt to reduce the part played by the CO shift reaction. For example, in Tart, K. R. et al., "Methanation Key to SNG Success", Hydrocarbon Processing, (April 1981), p. 114, et seq. there is disclosed a methanation process using a synthesis gas produced from coal gasification. A slagging gasifier is fed batchwise with a coal feed. A mixture of steam and oxygen are used in the gasifier to produce hot gaseous products The product gas from the slagging gasifier is said to have a hydrogen to carbon monoxide ratio of about 0.5. As the desired hydrogen to carbon monoxide ratio is generally between 1 and 3, the product gas from the gasifier may require a catalytic carbon monoxide shift conversion prior to chemical synthesis to obtain the desired hydrogen to carbon monoxide ratio. Alternatively, as disclosed in the article, CO-shift conversion and methane synthesis can be accomplished simultaneously over a single catalyst.
As indicated in that article, as a result of the process a separate CO shift stage with all its ancillary equipment is no longer needed. According to this method purified synthesis gas enters a saturator in which it is contacted with a countercurrent flow of water that has been heated by indirect heat exchange with gas streams within the process. This arrangement is said to utilize otherwise unusable low grade heat for provision of process steam and is also said to make a substantial contribution to the efficiency of the process. Saturated feed gas then undergoes direct methane synthesis in a series of catalytic reactors, the outlet temperatures of which are controlled by recycle of cooled product gas. The heat released from the highly exothermic synthesis reactions is used to generate high pressure steam for export to other process stages. Final methanation, gas cooling and carbon dioxide removal and drying are said to yield a synthetic natural gas containing less than 3% hydrogen and 0.1% carbon monoxide.
Another method produces methanol from gases produced in a Lurgi fixed bed gasification process. Coal, oxygen and steam are supplied to the gasifier in such quantities so as to cause the CO shift reaction to proceed inside the gasifier and to produce a crude gas in which the hydrogen to carbon monoxide ratio of the crude gas already meets the desired hyrogen to carbon monoxide ratio. The gas is cleaned, treated for removal of acid gases and is ultimately fed to a methanol reactor recycle loop. However, because the CO shift reaction occurs in the gasifier, without the aid of a catalyst, it is understood that the gasifier needs more steam than is required in a combination of slagging gasifier followed by steam addition prior to an external catalytic shift reactor.
The literature also discloses a number of processes using separation membranes in conjunction with chemical processes. For example, Null, U.S. Pat. No. 4,264,338 discloses a process for the separation of gases by selective membrane diffusion or permeation without increasing the work required. This is said to be accomplished by directing a permeate mixture from a second or later stage of separation to a recycle stage of membrane separation to provide a permeate enriched in the desired gas or gases. This step is followed by blending the permeate with the gaseous feed to the second or later stage of separation.
Null, et al., U.S. Pat. No. 4,180,553 discloses an ammonia synthesis process utilizing a separation membrane. In the process a purge stream from an ammonia synthesis loop is contacted at above atmospheric pressure with the feed side of a separation membrane which exhibits selected permeation of each of hydrogen and ammonia as compared to the permeation of each of methane and nitrogen. A total pressure differential across the membrane is maintained to provide a driving force for the permeation of hydrogen and ammonia through the membrane A hydrogen-rich permeating gas which contains ammonia is obtained on the permeate exit side of the membrane. Permeating gas is combined with the gas in the ammonia synthesis loop and passed to an ammonia reaction zone for conversion to ammonia.
Other methods use membrane separators in a variety of processes. For example, one process uses membranes to recover hydrogen and carbon oxides from a methanol synthesis purge stream leaving a methanol reactor. The hydrogen and oxides of carbon are recycled as feed for the methanol reactor.
These and other processes utilizing a feed stream comprising hydrogen and carbon monoxide to produce a hydrocarbon product or energy or both suffer from one or more of several defects or limitations. For example, prior processes using the shift reaction to obtain appropriate ratios of hydrogen to carbon monoxide introduce large amounts of carbon dioxide which subsequently must be removed from the system. Other processes require the use of purge steams taking off a material portion of reactants when recycle is used. Further, additional steam over and above that consumed in the reaction must often be generated. This is to force the equilibrium constant in favor of hydrogen and carbon dioxide; and, also to prevent excessive temperature rise across the reactor. Any recovered condensate containing acid gases must be subsequently purified prior to reuse in a steam generator.
These and other disadvantages or limitations are substantially minimized, if not eliminated, by the present invention.